During thermal therapy, energy is deposited into a target zone. Energy can be delivered as sonications in the form of focused ultrasound waves. Sonications are spaced in time by an intersonication delay to minimize thermal build-up. This delay is the cool-down period.
Document WO2010029474A1 describes an MR guided thermal treatment system in which the cool-down period is regulated in dependence of the off-focus maximum temperature during the energy deposit preceding the cool-down period. The maximum temperature rise in the off-focus region is approximately linearly dependent on the deposited energy density and a measurement of the maximum temperature can therefore be used to set the cool-down period. The linear dependence appears to be valid when the temperature decrease due to diffusion of heat can be neglected in the middle of the off-focus ultrasound cone during heating.
Partanen et al Reduction of peak acoustic pressure and shaping of heated region by use of multifoci sonications in MR-guided high-intensity focused ultrasound mediated mild hyperthermia, Med Phys 2013 40(1) is about mild hyperthermia, which is in the range of 40-45 degrees Celsius and to be used as an adjuvant for both radiotherapy and chemotherapy. In Partanen et al a binary control algorithm was used for real-time mild hyperthermia feed-back control (abstract, methods). The goal of Partanen et al is to keep temperatures inside the target within a range.
US 2012/101412A1 is about a thermal treatment method, wherein a temperature field in a region encompassing the target tissue and non-target tissue is monitored. Based on the monitored temperature field, the temperature in the non-target tissue is actively adjusted.
WO2011/021106A2 is about a method for performing proton resonance frequency based MR temperature measurements. WO2011/021106A2 tries to solve the problem that during MR thermal imaging a magnetic background field and the magnetic field gradient may change due to various factors unrelated to changes in temperature. These effects may result in incorrect temperature estimates.